1. Field of the Invention
The present invention relates to piezoelectric devices used as piezoelectric resonators and piezoelectric filters and to methods for manufacturing the same. In particular, the present invention relates to a piezoelectric device including a piezoelectric substrate having outer coating substrates secured on two surfaces thereof via adhesive layers, the piezoelectric substrate including an energy-trap-type piezoelectric vibration unit. The present invention also relates to a method for manufacturing such a piezoelectric device.
2. Description of the Related Art
In an energy-trap-type piezoelectric device, a piezoelectric vibration unit is provided partially on a piezoelectric substrate. The piezoelectric vibration unit must be sealed in a manner such that the piezoelectric vibration unit is not prevented from vibrating.
A piezoelectric device of the energy-trap-type described above is disclosed in Japanese Unexamined Patent Application Publication No. 8-335844. The piezoelectric device is described below with reference to FIGS. 4 and 5.
A piezoelectric device 51 shown in FIG. 4 includes a piezoelectric substrate 52 laminated with outer coating substrates 53 and 54 on the upper and lower surfaces thereof. A layered body defined by the piezoelectric substrate 52 and the outer coating substrates 53 and 54 is provided with external electrodes 55, 56, and 57.
As shown in FIG. 5, the piezoelectric substrate 52 includes driving electrodes provided partially on the upper and lower surfaces of the piezoelectric substrate 52, which define piezoelectric vibration units 52a and 52b. The outer coating substrates 53 and 54 are bonded to the piezoelectric substrate 52 via adhesive layers which have through-holes to provide spaces for the vibration of the piezoelectric vibration units 52a and 52b. 
In the piezoelectric device 51, the adhesive layers include first layers 58 and 59 which are located adjacent to the outer coating substrates 53 and 54, respectively, and second layers 60 and 61 which are disposed adjacent to the piezoelectric substrate 52. The first layers 58 and 59 include a soft adhesive having a Shore D hardness of not more than 60, and the second layers 60 and 61 include a hard adhesive having a Shore D hardness of at least 60. The first layers 58 and 59 are provided with through-holes 58a and 58b and through-holes 59a and 59b, respectively. The second layers 60 and 61 are provided with through-holes 60a and through-holes 61a, respectively.
An excellent damping effect and a space for vibration are provided by the first soft adhesive layers 58 and 59 and the second hard adhesive layers 60 and 61 laminated in the piezoelectric device 51, respectively. However, when the outer coating substrates 53 and 54 expand or contract, for example, by heating when mounting on a printed circuit board, the stress produced by the expansion or contraction is absorbed by the first layers 58 and 59, thereby suppressing variation in the frequency characteristics of the device after soldering and when kept in a humid environment.
The cut-away through-holes 58b and 59b are provided at the periphery of the first layers 58 and 59 for preventing the external electrodes 55, 56, and 57 from breaking. There is a risk of breakage of the external electrodes 55, 56, and 57 in that, when applying the first soft adhesive layers 58 and 59 to the outer coating substrate 53 and 54, the adhesive tends to flow out from the periphery of the outer coating substrate 53 and 54 to the side edge portions of the layered body due to its relative softness, and the flow of adhesive to the side edges portions of the layered body prevents the external electrodes 55, 56, and 57 from being properly formed by thin-film deposition or by coating and curing a conductive paste.
In the first step of manufacturing the piezoelectric device 51, the first layer 58 of a soft adhesive is provided on a surface of the planar outer coating substrate 53, and the first layer 59 of the soft adhesive is provided on a surface of the planar outer coating substrate 54. The viscosity of an adhesive having a Shore D hardness of not more than 60 is generally not more than 1.5xc3x97105 mPas at 25xc2x0 C. Therefore, in spite of being provided with the cut-away through-holes 58b and 59b, there is a risk of the adhesive flowing into the cut-away through-holes 58b and 59b, and flowing out of the periphery on which the external electrodes 55, 56, and 57 are provided, thereby causing a risk of breakage.
The above-described problem is eliminated when an adhesive having a Shore D harness of not more than 60 and having a viscosity before curing of at least approximately 3.0xc3x97105 mPas at 25xc2x0 C. is used, however, such an adhesive is difficult to produce.
When manufacturing the piezoelectric device 51, the second layers 60 and 61 of a hard adhesive are provided on the first layers 58 and 59. In this case, recesses are provided in the second layers 60 and 61 in the layered body by the cut-away through-holes 58b and 59b of the first layers 58 and 59 disposed under the second layers 60 and 61. Specifically, the thickness of the adhesive varies between a region at the periphery of the layered body on which the external electrodes 55, 56, and 57 are provided and the remaining region on which the external electrodes 55, 56, and 57 are not provided, which produces gaps at the periphery of the layered body, thereby causing a risk of breakage of the external electrodes 55, 56, and 57.
In order to overcome the problems described above, preferred embodiments of the present invention provide a reliable piezoelectric device and a method for manufacturing the same, in which a space for vibration of the piezoelectric vibration unit is provided, the stress produced by a variation in temperature during reflow soldering or other suitable methods is suppressed, the frequency characteristics are not altered, and the risk of breakage of external electrodes caused by adhesive layers is greatly reduced.
According to a preferred embodiment of the present invention, a piezoelectric device includes a piezoelectric substrate provided with driving electrodes on the major surfaces thereof and opposing each other, outer coating substrates, each of which is laminated on the major surfaces of the piezoelectric substrate, adhesive layers to bond the piezoelectric substrate and the outer coating substrates to each other at the major surfaces of the piezoelectric substrate, and a plurality of external electrodes provided on lateral side surfaces of a layered body including the piezoelectric substrate, the outer coating substrates, and the adhesive layers. Each of the adhesive layers includes a first layer of an adhesive having a Shore D hardness after curing of not more than approximately 60 which is arranged such that the first layer is not exposed in a region of the lateral side surfaces of the layered body having the plurality of external electrodes provided thereon, a second layer of an adhesive having a Shore D hardness after curing of at least approximately 60, and a third layer of an adhesive having a Shore D hardness after curing of at least approximately 60, and having a viscosity before curing of at least approximately 3.0xc3x97105 mPas at a temperature of 25xc2x0 C. The third layer is disposed between the periphery of the first layer and the region of the lateral side surfaces of the layered body which is provided with the plurality of external electrodes thereon. The second layer is laminated on the first layer and the third layer.
According to a preferred embodiment of the present invention, the first layer and the third layer of each of the adhesive layers are disposed adjacent to the outer coating substrate and the second layer of each of the adhesive layers is disposed adjacent the piezoelectric substrate.
In the piezoelectric device according a preferred embodiment of the present invention, the first layer of each of the adhesive layers is provided with through-holes at the lateral side portions of the layered body, and the third layer of each of the adhesive layers is disposed in these through-holes, such that the first layer is not exposed in a region of the lateral side portions of the layered body having the plurality of external electrodes provided thereon.
The piezoelectric device according to preferred embodiments of the present invention, which may be used as various piezoelectric devices such as a piezoelectric resonator, a piezoelectric filter, or other suitable devices, may include a plurality of the driving electrodes to produce a piezoelectric filter.
According to another preferred embodiment of the present invention, a method for manufacturing a piezoelectric device is provided. The piezoelectric device including a piezoelectric substrate provided with driving electrodes on the major surfaces thereof and opposing each other, outer coating substrates, each of which is laminated on the major surfaces of the piezoelectric substrate, adhesive layers to bond the piezoelectric substrate and the outer coating substrates to each other at the major surfaces of the piezoelectric substrate, and a plurality of external electrodes provided on lateral side surfaces of a layered body including the piezoelectric substrate, the outer coating substrates, and the adhesive layers. Each of the adhesive layers includes a first layer of an adhesive having a Shore D harness after curing of not more than approximately 60 which is disposed such that the first layer is not exposed in a region of the lateral side surfaces of the layered body having the plurality of external electrodes provided thereon, a second layer of an adhesive having a Shore D hardness after curing of at least approximately 60, and a third layer of an adhesive having a Shore D hardness, after curing, of at least approximately 60, and having a viscosity before curing of not less than approximately 3.0xc3x97105 mPas at a temperature of 25xc2x0 C., the third layer being disposed between the periphery of the first layer and the region of the lateral side surfaces of the layered body having the plurality of external electrodes provided thereon. The second layer is laminated on the first layer and the third layer. The method for manufacturing the piezoelectric device includes the steps of forming the third layer of each of the adhesive layers on a major surface of each of the outer coating substrates, forming the first layer of each of the adhesive layers in a region of the major surface of each of the outer coating substrates which is not provided with the third layer, preparing the second layer of each of the adhesive layers, and bonding, via the second layer, the piezoelectric substrate and the outer coating substrates to each other, the outer coating substrates being provided with the third layer and the first layer.
Other features, elements, characteristics and advantages of the present invention will become apparent from the detailed description of preferred embodiments thereof with reference to the drawings attached hereto.